A comparison of drift wave stability in stellarator and tokamak geometry

The influence of plasma geometry on the linear stability of electrostatic ion-temperature-gradient driven drift modes (ITG or eta(i)=L-n/L-Ti modes) is investigated. An advanced fluid model is used for the ions together with Boltzmann distributed electrons. The derived eigenvalue equation is solved numerically. A comparison is made between an H-1NF [Fusion Technol. 17, 123 (1990)] like stellarator equilibrium, a numerical tokamak equilibrium and the analytical (s) over cap-alpha equilibrium. The numerical and the analytical tokamak are found to be in good agreement in the low inverse aspect ratio limit. The growth rates of the tokamak and stellarator are comparable whereas the modulus of the real frequency is substantially larger in the stellarator. The threshold in eta(i) for the stellarator is found to be somewhat larger. In addition, a stronger stabilization of the ITG mode growth is found for large epsilon(n)(=L-n/R) in the stellarator case. (C) 2002 American Institute of Physics.